1. Field of the Invention
The present invention relates to a liquid crystal display device of negative display type having a light shielding layer formed therein.
2. Discussion of Background
Heretofore, a negative display wherein letters or patterns are displayed brightly on a dark display surface has been used in many cases for liquid crystal display devices used for the display portions of watches or meters for automobiles.
In a negative liquid crystal display device, no voltage is applied to the liquid crystal layer at the background area and liquid crystal molecules are twisted at the background area, whereby light propagates along the twisted liquid crystal molecules, and a pair of polarizing films are disposed with their polarization axes being in parallel with each other so that no light is transmitted at the background area. At the background area of a usual negative liquid crystal display device, light propagates in a twisted fashion as mentioned above, but there has been a problem such that light having a certain color is transmitted to some extent, since the transmittance of the conventional negative liquid crystal display device depends on wavelength of the light. The ratio in contrast (contrast ratio) between a case where a pair of polarizing films are simply disposed with their polarization axes being perpendicular to each other and a case wherein they are disposed with their axes being in parallel with each other, will be as high as at least 1,000. However, when a liquid crystal layer is sandwiched therebetween in a usual negative liquid crystal display device, light propagates in a twisted fashion at the background area, as mentioned above, whereby no adequate polarization is obtainable with respect to all the colors and leakage of light is observed at the background area, which used to be a problem.
It has been proposed to form a light shielding layer at the background area thereby to prevent the leakage of light at the background area.
However, when such a light shielding layer is provided simply to prevent the leakage of light at the background area, the background area can be made dark, but leakage of light is still observed at a non-display (dark) pattern portion within the display pattern area, where no light transmission is desired and the same problem as mentioned above with respect to the background area will result at this portion. If the ratio in the darkness between the background area and this non-display pattern portion becomes substantial, the display may likely be misread as light is transmitted at the portion where no light should properly be transmitted.
It has been previously proposed to provide a light shielding layer at the background area and at the same time dispose polarizing films in the same manner as in the case of a positive liquid crystal display device so that light is transmitted at a portion where no voltage is applied to liquid crystal, whereby the operation of the display pattern is conducted by applying a voltage to the liquid crystal at a portion where no light is to be transmitted. Thus, at a portion where a voltage is applied to the liquid crystal, the liquid crystal molecules are vertically aligned, whereby there will be no influence by a color, and by disposing a pair of polarizing films so that their polarization axes are perpendicular to each other, it is possible to obtain a high level of light shielding effect by adequately utilizing the polarizing performance of the polarizing films, and it is possible to obtain a high contrast of at least 1,000.
Thus, with a liquid crystal display device having a light shielding layer formed therein and having polarizing films with their polarization axes arranged in the same manner as in a positive display device, it is possible to obtain a high contrast as mentioned above, when observed from the front. However, when observed from an oblique direction, there will be a phenomenon in which leakage of light is observed to some extent at a non-display (dark) pattern portion where a voltage is applied to the liquid crystal, due to the viewing angle dependency of the liquid crystal. Consequently, the difference in the light transmittance will be distinct between the portion where the light transmittance is almost completely shielded by the light shielding layer and the non-display pattern portion where a voltage is applied to the liquid crystal, whereby misreading is likely to be led.
Therefore, such a liquid crystal device is useful for an application where a viewing angle is restricted to a front direction or to a certain specific direction, but it has a problem that the viewing angle is narrow for an application where a wide range of viewing angle is required.